Acoustic sensors are largely based upon electronic piezoelectric devices where deformation of the piezoelectric material results in a voltage change which can be measured using suitable electronics. However, these devices require essentially local instrumentation which is a disadvantage for remote sensing applications such as hydrophones deployed in underwater arrays and results in sensors which are bulkier and more complex tan desired. This is due to the data and power cabling and pre-amp requirements which make hydrophone arrays of this nature difficult to deploy and maintain. Some other disadvantages with piezoelectric based devices include their susceptibility to electromagnetic interference thereby reducing their overall sensitivity and the fact that due to their active electronics they may be detected by other parties.
To this end there have been a number of attempts to develop acoustic sensors based on fibre optic technology (see for example C K Kirkendall and A Dandridge, “Overview of High Performance Fibre-Optic Sensing”, J. Phys. D: Appl. Phys. 37, R197-R216, 2004). One attempt employs a distributed feedback fibre laser (DFB FL) whose characteristics include a very narrow lasing wavelength output and the ability to be configured to operate at different wavelengths, making them suitable for wavelength division multiplexing (see for example D J Hill, P J Nash, D A Jackson, D J Webb, S F O'Neill, I Bennion and I Zhang, “A Fibre Laser Hydrophone Array”, Proc. SPIE, 3860, 55-66, 1999).
These sensors are based on an important feature of these lasers in that the frequency (or equivalently wavelength) of laser light emitted is sensitive to the induced stain on the fibre. This is understood to arise from changes in the resonant cavity size of the fibre laser and additionally the refractive index of the fibre in those regions which are under strain. However, whilst these devices offer a number of advantages over electronic based devices due to the lack of electronic instrumentation required at the “wet end” and furthermore the ability to multiplex a number of sensors in a single fibre, they have been found to be insufficiently sensitive for a number of hydrophone type applications of interest. Also, they have been found to be inherently sensitive to non-acoustic vibrations making them unacceptably noisy for many applications.
Some attempts to increase the sensitivity of a distributed feedback fibre laser include encapsulating the fibre in a cylinder of epoxy or polyurethane thereby forming a mandrel surrounding the laser active region of the fibre laser. Whilst the increased bulk of the fibre surrounding the cavity and associated regions of the distributed feedback fibre laser improves the strain to pressure sensitivity somewhat it is still insufficient for those applications where extreme sensitivity is required such as a hydrophone. In addition, distributed feedback fibre lasers which have been modified in this manner suffer from overall lower resonant frequencies due to the increased mass of encapsulating material that is used to increase the strain to pressure sensitivity.
There have also been attempts in the prior art to enhance sensitivity by attaching the two ends of the fibre laser to a mechanical structure so that the fibre is under tension similar to a guitar string. This structure is configured to elongate or compress in response to pressure changes, thereby straining the fibre. These devices only address the issue of enhanced pressure sensitivity and do not solve the equally important problem of inherent vibrational noise sensitivity and in fact in some circumstances they function to increase noise sensitivity.
It is an object of the present invention to provide an acoustic sensor based on fibre optic laser technology suitable for deployment as a hydrophone.